Photovoltaic modules for converting solar energy to electrical energy generally are made up of a set of solar cells which are mounted on a common base and are electrically interconnected. Frequently, a plurality of these photovoltaic modules are connected to obtain a desired electrical output, i.e. a specific voltage and a specific current.
Photovoltaic modules are susceptible to failure and loss of conversion efficiency during operation due to degradation and/or short circuit damage of individual solar cells within the modules. As the current carrying capacity of a particular solar cell decreases, the output from other solar cells connected in series with this cell reverse biases the damaged cell. The voltage across the damaged cell increases in a reverse polarity until the full output voltage of all other solar cells connected in serial arrangement with the damaged cell is impressed on the damaged cell. This causes the damaged cell to break down at a relatively high reverse breakdown voltage. The damaged cell then dissipates a large amount of power, leading to a considerable amount of heat to be generated in this specific solar cell. This
May cause major problems and may even lead to the photovoltaic module catching fire.
This problem is generally resolved by bridging the serial arrangement of solar cells with a diode in such a way that the diode's cathode is connected to the positive terminal of the serial string of solar cells and the diode's anode is connected to the negative terminal of the serial string. If one of the solar cells is defective, the diode provides a low impedance bypass of the serial string of solar cells. As a consequence, the power being dissipated in the damaged cell is limited to, at most, the power generated by the undamaged cells in the serial string.
A diode connected in parallel with a serial string of solar cells thus prevents overheating in case of damage of one (or several) of the solar cells by limiting power dissipation in the location of the damaged cell(s). However, the undamaged cells are also bypassed, and thus the performance of the photovoltaic module as a whole is reduced. A setup circumventing this problem is described in U.S. Pat. No. 6,020,555 A: In this setup, each solar cell in the serial arrangement is provided with a diode of its own which is connected in parallel with this solar cell; in case of a solar cell failure, the damaged solar cell is bypassed by its diode while the other solar cells remain in operation. The setup disclosed in U.S. Pat. No. 6,020,555 A thus protects solar cells connected in series against failure due to mechanical damage or shadowing of individual solar cells while maintaining full output of the remaining cells.
A scheme for protecting a series parallel arrangement of solar cells against shadowing and cell defects is described in US 2009/0014056 A1. As illustrated in FIG. 15 of US 2009/0014056 A1, a set of reverse current protection diodes and bypass diodes are periodically located in an array of solar cells interconnected by parallel and serial connections. Parallel connections between the solar cells provide current bypasses around single cells in the array that may have low performance due to manufacturing defects or shading, and bypass diodes route current around rows of cells that have a low performance or are shadowed. If excessive current flows through the parallel or series electrical connections, the metals of these connections heat due to ohmic energy dissipation; this may lead to a melting of the metal and permanently open the parallel circuit connection. This kind of open circuit fusing between solar cells connected in parallel can be used to permanently open the circuit around individual solar cells that have a low performance.
While the protective setup described in US 2009/0014056 A1 is capable of permanently short-circuiting defective solar cells within a photovoltaic module, this is accomplished by destroying electrical connections within the module which may be needed otherwise. Thus, there is a need for an overheat protection system which temporarily or permanently eliminates a defective solar cell of a serial and/or parallel array while leaving the remaining solar cell network intact.